Like curing meats, the practice of salting duck eggs may have started as a method of preservation, but now salted duck eggs are a delicacy. Salting makes the egg whites dense and almost rubber-eraser-like in appearance, but it's the yolks that are especially prized. There's nothing quite like a good salted duck egg yolk. If properly salted, the duck egg yolks are creamy, granular, and oily all at once—an intriguing textural composition that tastes especially rich and salty.
In the countryside it was the practice to cover the eggs in a mixture of salt and mud, but nowadays it's more common to find homemade salted duck eggs seeped in a simple brine of water and salt. Moreover, the ducks eggs can be bought already salted, cooked, and vacuum-packed at most Chinese or Vietnamese grocery stores. They're common appetizers or just meal accouterments in Chinese cuisine.
Hard boiled, the eggs can be eaten halved or quartered right off the shell. As a kid I always ate sections of salted duck egg with my porridge in the mornings. I gobbled up the egg yolk on the first day but my mother would make me eat the egg white in subsequent days. It would take me several mornings of eating and complaining to finish the egg white, which seemed so dull in comparison to the yolk.
Salted duck egg yolks are a versatile cooking ingredient. You'll find them as an ingredient in the center of glutinous rice patties wrapped in bamboo or banana leaves. The egg yolks are also used as a filling in Phoenix buns, in which they're combined with condensed milk for a salty-sweet mixture that oozes out of the steamed buns.
Added to a stir-fried dish, the duck egg yolks provide an instant sauce, coating whatever you cook in a layer of rich, fatty yolk paste. As soon as the egg yolks hit the wok, they rise in a golden froth that has Chinese restaurant calling the dishes some version of "gold" or "golden-sand." It's probably one of the more indulgent ways to eat a vegetable.